Drive mechanism for a drug delivery device and drug delivery device

ABSTRACT

A lead screw, a lead screw nut and a drive member are aligned with an axis defining an axial direction and an opposite axial direction. A coupling between the lead screw and the lead screw nut allows a helical movement of the lead screw with respect to the lead screw nut at least in the axial direction. The lead screw is coupled with the drive member, the coupling generating a helical movement of the lead screw with respect to the drive member when the drive member is moved in the axial direction with respect to the lead screw. A dispense stop feature of the lead screw nut and a dispense stop feature of the drive member prevent the generation of the helical movement of the lead screw when a specified end position of the drive member is approached.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase Application pursuant to35 U.S.C. §371 of International Application No. PCT/EP2011/067418 filedOct. 5, 2011, which claims priority to European Patent Application No.10186736.4 filed Oct. 6, 2010. The entire disclosure contents of theseapplications are herewith incorporated by reference into the presentapplication.

FIELD OF DISCLOSURE

The present invention relates to a drive mechanism for a drug deliverydevice, especially for a device that is designed for the delivery offixed doses.

BACKGROUND

Portable drug delivery devices are used for the administration of a drugthat is suitable for self-administration by a patient. A drug deliverydevice is especially useful in the shape of a pen, which can be handledeasily and kept everywhere available. A drug is delivered by means of adrive mechanism, which may also serve to set the dose to be delivered. Atype of drug delivery device is constructed to be refillable and thusreusable many times.

DE 102 37 258 B4 describes a drug delivery device in the shape of aninjection pen, which has a drive mechanism with elements that arerotated relatively to one another around a common axis.

SUMMARY

It is an object of the present invention to disclose a new drivemechanism for a drug delivery device and a drug delivery devicecomprising a new drive mechanism.

This object is achieved by a drive mechanism according to claim 1 and adrug delivery device according to claim 10. Further objects are achievedby embodiments according to the dependent claims.

The drive mechanism for a drug delivery device comprises a lead screw, alead screw nut and a drive member, aligned with an axis defining anaxial direction and an opposite axial direction. A coupling between thelead screw and the lead screw nut allows a helical movement of the leadscrew with respect to the lead screw nut at least in the axialdirection. The lead screw is coupled with the drive member, the couplinggenerating a helical movement of the lead screw with respect to thedrive member when the drive member is moved in the axial direction withrespect to the lead screw. The coupling is overridden to prevent ahelical movement of the lead screw with respect to the drive member whenthe drive member is moved in the opposite axial direction with respectto the lead screw. A dispense stop feature of the lead screw nut and adispense stop feature of the drive member interact and thereby preventthe generation of the helical movement of the lead screw when aspecified end position of the drive member is approached.

In an embodiment of the drive mechanism the dispense stop features havecorresponding contact surfaces, which are oblique to the axis.

Preferably, a generation of the helical movement of the lead screw isprevented by an interaction of the oblique contact faces. As an example,an interaction comprises a sliding movement of at least one of theoblique contact faces on the other oblique contact face. In particular,the drive mechanism may be configured such that the oblique contact faceof the dispense stop feature of the drive member is enabled to slidealong the oblique contact face of the dispense stop feature of the leadscrew nut. Thereby, a rotation of the drive member may be caused, inparticular a rotation of the drive member relative to the lead screwnut. An inclination of at least one of the oblique contact faces maycorrespond to the pitch of a thread of the drive member engaging withthe lead screw.

In a further embodiment of the drive mechanism the dispense stopfeatures have corresponding contact surfaces, which are provided as endfaces and inhibit a rotation of the drive member with respect to thelead screw nut at least in one direction.

In a further embodiment of the drive mechanism the dispense stop featureof the lead screw nut has the shape of a prism or truncated prism.

In a further embodiment of the drive mechanism the dispense stopfeatures guide the drive member with respect to the lead screw nut in ahelical movement having the same pitch as the helical movement of thelead screw with respect to the drive member.

As an example, such a movement may be generated by the interaction ofoblique contact faces of the dispense stop features, in particular bythe interaction of oblique contact faces having an inclinationcorresponding to a pitch of a thread of the drive member engaging withthe lead screw.

A further embodiment of the drive mechanism comprises a flexible guidefeature of the lead screw and a screw thread of the drive member. Thecoupling of the lead screw with the drive member is provided by theflexible guide feature engaging the screw thread. The screw thread hasthe same pitch as the helical movement of the lead screw with respect tothe drive member.

A further embodiment of the drive mechanism comprises stop features ofthe lead screw, which inhibit the helical movement of the lead screwwhen the drive member is moved in the opposite axial direction withrespect to the lead screw.

In a further embodiment of the drive mechanism the dispense stop featureof the lead screw nut is formed as an integral part of the lead screwnut, and the dispense stop feature of the drive member is formed as anintegral part of the drive member.

In a further embodiment of the drive mechanism the drive member and thelead screw nut are rotationally locked.

A drug delivery device that is provided with the drive mechanism maycomprise a body, which has a distal end and a proximal end, which arespaced apart in the direction of the axis of the drive mechanism.

The body can be any housing or any component that forms part of ahousing, for example. The body can also be some kind of an insertconnected with an exterior housing. The body may be designed to enablethe safe, correct, and/or easy handling of the device and/or to protectit from harmful liquids, dust or dirt. The body can be unitary or amultipart component of tubular or non-tubular shape. The body may housea cartridge, from which doses of a drug can be dispensed. The body canespecially have the shape of an injection pen.

The term “distal end” refers to a part of the body or housing which isintended to be arranged at a portion of the drug delivery device fromwhich a drug is dispensed. The term “proximal end” refers to a part ofthe body or housing which is remote from the distal end. The term“distal direction” refers to a movement in the same direction as amovement from the proximal end towards the distal end, not specifying apoint of departure nor an end point, so that the movement may go beyondthe distal end. The term “proximal direction” refers to a movement inthe direction opposite to the distal direction.

The term “lead screw” encompasses any element, whether unitary or ofmultipart construction, that is provided to transfer a movement to apiston, thus working as a piston rod, especially for the purpose ofdispensing a drug. The lead screw may be flexible or not.

The drive mechanism can be used to expel a drug from a receptacle orcartridge inserted in the body of a drug delivery device. The drugdelivery device can be a disposable or re-usable device designed todispense a dose of a drug, especially a liquid, which may be insulin, agrowth hormone, a heparin, or an analogue and/or a derivative thereof,for example. The drug may be administered by a needle, or the device maybe needle-free. The device may be further designed to monitorphysiological properties like blood glucose levels, for example. Eachtime the lead screw is shifted in the distal direction with respect tothe body, a certain amount of the drug is expelled from the drugdelivery device.

The term “drug”, as used herein, preferably means a pharmaceuticalformulation containing at least one pharmaceutically active compound,

wherein in one embodiment the pharmaceutically active compound has amolecular weight up to 1500 Da and/or is a peptide, a proteine, apolysaccharide, a vaccine, a DNA, a RNA, an enzyme, an antibody, ahormone or an oligonucleotide, or a mixture of the above-mentionedpharmaceutically active compound,

wherein in a further embodiment the pharmaceutically active compound isuseful for the treatment and/or prophylaxis of diabetes mellitus orcomplications associated with diabetes mellitus such as diabeticretinopathy, thromboembolism disorders such as deep vein or pulmonarythromboembolism, acute coronary syndrome (ACS), angina, myocardialinfarction, cancer, macular degeneration, inflammation, hay fever,atherosclerosis and/or rheumatoid arthritis,

wherein in a further embodiment the pharmaceutically active compoundcomprises at least one peptide for the treatment and/or prophylaxis ofdiabetes mellitus or complications associated with diabetes mellitussuch as diabetic retinopathy,

wherein in a further embodiment the pharmaceutically active compoundcomprises at least one human insulin or a human insulin analogue orderivative, glucagon-like peptide (GLP-1) or an analogue or derivativethereof, or exedin-3 or exedin-4 or an analogue or derivative ofexedin-3 or exedin-4.

Insulin analogues are for example Gly(A21), Arg(B31), Arg(B32) humaninsulin; Lys(B3), Glu(B29) human insulin; Lys(B28), Pro(B29) humaninsulin; Asp(B28) human insulin; human insulin, wherein proline inposition B28 is replaced by Asp, Lys, Leu, Val or Ala and wherein inposition B29 Lys may be replaced by Pro; Ala(B26) human insulin;Des(B28-B30) human insulin; Des(B27) human insulin and Des(B30) humaninsulin.

Insulin derivates are for example B29-N-myristoyl-des(B30) humaninsulin; B29-N-palmitoyl-des(B30) human insulin; B29-N-myristoyl humaninsulin; B29-N-palmitoyl human insulin; B28-N-myristoyl LysB28ProB29human insulin; B28-N-palmitoyl-LysB28ProB29 human insulin;B30-N-myristoyl-ThrB29LysB30 human insulin; B30-N-palmitoyl-ThrB29LysB30human insulin; B29-N-(N-palmitoyl-Y-glutamyl)-des(B30) human insulin;B29-N-(N-lithocholyl-Y-glutamyl)-des(B30) human insulin;B29-N-(ω-carboxyheptadecanoyl)-des(B30) human insulin andB29-N-(w-carboxyheptadecanoyl) human insulin.

Exendin-4 for example means Exendin-4(1-39), a peptide of the sequenceH-His-Gly-Glu-Gly-Thr-Phe-Thr-Ser-Asp-Leu-Ser-Lys-Gln-Met-Glu-Glu-Glu-Ala-Val-Arg-Leu-Phe-Ile-Glu-Trp-Leu-Lys-Asn-Gly-Gly-Pro-Ser-Ser-Gly-Ala-Pro-Pro-Pro-Ser-NH2.

Exendin-4 derivatives are for example selected from the following listof compounds:

-   -   H-(Lys)4-des Pro36, des Pro37 Exendin-4(1-39)-NH2,    -   H-(Lys)5-des Pro36, des Pro37 Exendin-4(1-39)-NH2,    -   des Pro36 [Asp28] Exendin-4(1-39),    -   des Pro36 [IsoAsp28] Exendin-4(1-39),    -   des Pro36 [Met(O)14, Asp28] Exendin-4(1-39),    -   des Pro36 [Met(O)14, IsoAsp28] Exendin-4(1-39),    -   des Pro36 [Trp(O2)25, Asp28] Exendin-4(1-39),    -   des Pro36 [Trp(O2)25, IsoAsp28] Exendin-4(1-39),    -   des Pro36 [Met(O)14 Trp(O2)25, Asp28] Exendin-4(1-39),    -   des Pro36 [Met(O)14 Trp(O2)25, IsoAsp28] Exendin-4(1-39); or    -   des Pro36 [Asp28] Exendin-4(1-39),    -   des Pro36 [IsoAsp28] Exendin-4(1-39),    -   des Pro36 [Met(O)14, Asp28] Exendin-4(1-39),    -   des Pro36 [Met(O)14, IsoAsp28] Exendin-4(1-39),    -   des Pro36 [Trp(O2)25, Asp28] Exendin-4(1-39),    -   des Pro36 [Trp(O2)25, IsoAsp28] Exendin-4(1-39),    -   des Pro36 [Met(O)14 Trp(O2)25, Asp28] Exendin-4(1-39),    -   des Pro36 [Met(O)14 Trp(O2)25, IsoAsp28] Exendin-4(1-39),    -   wherein the group -Lys6-NH2 may be bound to the C-terminus of        the Exendin-4 derivative;    -   or an Exendin-4 derivative of the sequence    -   H-(Lys)6-des Pro36 [Asp28] Exendin-4(1-39)-Lys6-NH2,    -   des Asp28 Pro36, Pro37, Pro38Exendin-4(1-39)-NH2,    -   H-(Lys)6-des Pro36, Pro38 [Asp28] Exendin-4(1-39)-NH2,    -   H-Asn-(Glu)5des Pro36, Pro37, Pro38 [Asp28] Exendin-4(1-39)-NH2,    -   des Pro36, Pro37, Pro38 [Asp28] Exendin-4(1-39)-(Lys)6-NH2,    -   H-(Lys)6-des Pro36, Pro37, Pro38 [Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-(Lys)6-des Pro36 [Trp(02)25, Asp28] Exendin-4(1-39)-Lys6-NH2,    -   H-des Asp28 Pro36, Pro37, Pro38 [Trp(02)25] Exendin-4(1-39)-NH2,    -   H-(Lys)6-des Pro36, Pro37, Pro38 [Trp(02)25, Asp28]        Exendin-4(1-39)-NH2,    -   H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Trp(02)25, Asp28]        Exendin-4(1-39)-NH2,    -   des Pro36, Pro37, Pro38 [Trp(02)25, Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-(Lys)6-des Pro36, Pro37, Pro38 [Trp(02)25, Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Trp(02)25, Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-(Lys)6-des Pro36 [Met(0)14, Asp28] Exendin-4(1-39)-Lys6-NH2,    -   des Met(0)14 Asp28 Pro36, Pro37, Pro38 Exendin-4(1-39)-NH2,    -   H-(Lys)6-desPro36, Pro37, Pro38 [Met(0)14, Asp28]        Exendin-4(1-39)-NH2,    -   H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Met(0)14, Asp28]        Exendin-4(1-39)-NH2,    -   des Pro36, Pro37, Pro38 [Met(0)14, Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-(Lys)6-des Pro36, Pro37, Pro38 [Met(0)14, Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-Asn-(Glu)5 des Pro36, Pro37, Pro38 [Met(O)14, Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-Lys6-des Pro36 [Met(O)14, Trp(02)25, Asp28]        Exendin-4(1-39)-Lys6-NH2,    -   H-des Asp28 Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25]        Exendin-4(1-39)-NH2,    -   H-(Lys)6-des Pro36, Pro37, Pro38 [Met(O)14, Asp28]        Exendin-4(1-39)-NH2,    -   H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25,        Asp28] Exendin-4(1-39)-NH2,    -   des Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25, Asp28]        Exendin-4(1-39)-(Lys)6-NH2,    -   H-(Lys)6-des Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25, Asp28]        Exendin-4(S1-39)-(Lys)6-NH2,    -   H-Asn-(Glu)5-des Pro36, Pro37, Pro38 [Met(O)14, Trp(O2)25,        Asp28] Exendin-4(1-39)-(Lys)6-NH2;        or a pharmaceutically acceptable salt or solvate of any one of        the afore-mentioned Exedin-4 derivative.

Hormones are for example hypophysis hormones or hypothalamus hormones orregulatory active peptides and their antagonists as listed in RoteListe, ed. 2008, Chapter 50, such as Gonadotropine (Follitropin,Lutropin, Choriongonadotropin, Menotropin), Somatropine (Somatropin),Desmopressin, Terlipressin, Gonadorelin, Triptorelin, Leuprorelin,Buserelin, Nafarelin, Goserelin.

A polysaccharide is for example a glucosaminoglycane, a hyaluronic acid,a heparin, a low molecular weight heparin or an ultra low molecularweight heparin or a derivative thereof, or a sulphated, e.g. apoly-sulphated form of the above-mentioned polysaccharides, and/or apharmaceutically acceptable salt thereof. An example of apharmaceutically acceptable salt of a poly-sulphated low molecularweight heparin is enoxaparin sodium.

Pharmaceutically acceptable salts are for example acid addition saltsand basic salts. Acid addition salts are e.g. HCl or HBr salts. Basicsalts are e.g. salts having a cation selected from alkali or alkaline,e.g. Na+, or K+, or Ca2+, or an ammonium ion N+(R1)(R2)(R3)(R4), whereinR1 to R4 independently of each other mean: hydrogen, an optionallysubstituted C1-C6-alkyl group, an optionally substituted C2-C6-alkenylgroup, an optionally substituted C6-C10-aryl group, or an optionallysubstituted C6-C10-heteroaryl group. Further examples ofpharmaceutically acceptable salts are described in “Remington'sPharmaceutical Sciences” 17. ed. Alfonso R. Gennaro (Ed.), MarkPublishing Company, Easton, Pa., U.S.A., 1985 and in Encyclopedia ofPharmaceutical Technology.

Pharmaceutically acceptable solvates are for example hydrates.

In the following, a more detailed description of examples andembodiments of the drive mechanism is given in conjunction with theappended figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a perspective view of a cross-section of an injection pencomprising an embodiment of the drive mechanism.

FIG. 2 shows a detailed view of the dispense stop features of the drivemember and the lead screw nut.

FIG. 3 shows an enlarged view of the distal end of the lead screw.

DETAILED DESCRIPTION

FIG. 1 shows a cut-away view of an injection pen comprising the drivemechanism. The drive mechanism is arranged in a body 1 having a distalend 2 and a proximal end 3. A lead screw 5 is arranged along an axis 4of the device. A screw thread 6 of the lead screw 5 is coupled to adrive feature of a lead screw nut 7 engaging the screw thread 6, inorder to guide a helical movement of the lead screw 5 with respect tothe lead screw nut 7. In further embodiments, the screw thread and thedrive feature can be reversed such that the lead screw is provided withdiscrete drive features and the lead screw nut is provided with ahelical screw thread. The lead screw nut 7 is rotationally locked to thebody 1.

The embodiment shown in FIG. 1 comprises a drive member 8, which can beoperated by the user by means of a button 9, which is arranged at theproximal end 3 and juts out of the body 1. The drive member 8 is coupledor engaged with the lead screw 5. This is achieved, in this embodiment,by means of a screw thread 18 of the drive member 8 and a flexible guidefeature 15 of the lead screw 5. The drive member 8 can especially be adrive sleeve of essentially cylindrical shape, the axis of the drivesleeve being arranged parallel to the axis 4 of the device. The leadscrew 5 may be disposed to enter the drive member 8.

A removable and attachable part 11 of the body 1 may be provided as acartridge holder. When this part 11 is removed from the rest of the body1, a cartridge 12 can be inserted. When the part 11 is attached to thebody 1, the lead screw 5 is brought into contact with a piston 13, whichis provided to expel a drug from the cartridge 12. A bearing 14 may bearranged between the lead screw 5 and the piston 13 in order to preventany damage that might be caused by a relative movement between the leadscrew 5 and the piston 13. The lead screw 5 functions as a piston rod toadvance the piston 13 in the distal direction.

During a delivery operation, the lead screw 5 is helically moved in thedistal direction with respect to the body 1. The lead screw 5 is guidedby the lead screw nut 7, which is engaged with the screw thread 6 of thelead screw 5. Stop features 17 (shown in FIG. 3 described below) areprovided in the screw thread 6 of the lead screw 5 to enable a setoperation, by which a fixed dose that is to be dispensed can be preset.For this purpose, the drive member 8 is drawn in the proximal directionrelatively to the body 1 and to the lead screw 5. The drive member 8 iscoupled with the lead screw 5. In the embodiment shown in FIG. 1, thecoupling is achieved with the screw thread 18 of the drive member 8 andthe flexible guide feature 15 of the lead screw 5. During the setoperation, the lead screw 5 must not be moved. Therefore, the engagementbetween the drive member 8 and the lead screw 5 is temporarily releasedduring the set operation. This may be achieved by a deformation of theflexible guide feature 15 to override the screw thread 18 of the drivemember 8. In spite of the engagement between the drive member 8 and thelead screw 5, the drive member 8 can therefore be moved without beingrotated, while the lead screw 5 stays stationary with respect to thebody. Overriding the engagement between the drive member 8 and the leadscrew 5 is facilitated by flexible guide features 15, which can be benttowards the central axis 4. A rotation of the drive member 8 withrespect to the body 1 may be prevented by guide features 10, which maybe protruding elements of the body 1 engaging an axial groove in theouter surface of the drive member 8, for instance.

After the drive member 8 has been moved a distance corresponding to thepitch of the screw thread 18 of the drive member 8, the flexible guidefeature 15 of the lead screw 5 reengages the screw thread 18 of thedrive member 8, and the user can advance the lead screw 5 by pushing thedrive member 8 back in the distal direction. This method of operation bydisengaging and reengaging the lead screw 5 with the drive member 8relies entirely on the lead screw 5 remaining substantially stationaryduring the setting operation. Should the lead screw 5 rotate or moveaxially during setting, then the drive member 8 would very likely notcorrectly reengage with the lead screw 5 and thus cause dose inaccuracy.Therefore, the lead screw nut 7 guiding the helical movement of the leadscrew 5 with respect to the body 1 is rotationally locked to the body 1at least during the dispense operation and, furthermore, the lead screw5 is provided with stop features interfering with the rotation of thelead screw 5 in such a manner that the rotation is inhibited in thepositions of the lead screw 5 which are obtained after the drug deliveryand before the setting of a new dose. The rotation of the lead screw 5is thus locked with respect to the lead screw nut 7, and the lead screwnut 7 is prevented from rotating relatively to the body 1. Therefore,when the drive member 8 is drawn in the proximal direction, the relativelinear motion between the drive member 8 and the lead screw 5 causes theengagement of the drive member and the stationary lead screw 5 to beoverridden and thus the engagement between the drive member 8 and thelead screw 5 to be released. The stop features are therefore preferablyarranged at least on the distal sidewall of the screw thread 6 of thelead screw 5, while the screw thread 6 may be smooth, forming a helix,on its proximal sidewall. When the drive member 8 is pushed in thedistal direction, a guide means of the lead screw nut 7 engaging thescrew thread 6 of the lead screw 5 stays in contact with the smoothproximal sidewall of the screw thread 6, thus enabling a smooth helicalmovement of the lead screw 5 sliding through the opening of the leadscrew nut 7. Therefore, the stop features do not interfere with therelative motion of the lead screw 5 with respect to the lead screw nut 7during the dispense operation.

The stop features may especially be provided by recesses of a helicalgroove forming the screw thread 6 of the lead screw 5. The recesses canhave contact faces arranged transverse to the axis 4 and interruptingthe smooth helix of the relevant sidewall of the groove forming thescrew thread 6. The contact faces may especially be flat portions,essentially perpendicular to the axis 4 or at least having zero helixangle, but may comprise a rake angle in the radial direction. A drivefeature of the lead screw nut 7 may be formed in such a manner that itenters the recesses and stops on the contact face. When the drivefeature of the lead screw nut 7 comes into contact with one of the flatportions, the generally perpendicular orientation of the flat portionwith respect to the axis 4 causes the guidance of the helical movementof the lead screw 5 with respect to the body 1 to be stopped. It may befavorable if the drive feature of the lead screw nut 7 that engages withthe screw thread 6 of the lead screw 5 and is stopped in the recesses ismade up of one or more individual drive features and is not formed by acompletely continuous helix. The stop features are arranged in such afashion that, after a dose of the drug has been fully delivered and thedevice is ready for the next dose to be set, one of the stop features isin a position ready to stop the rotation of the lead screw 5 when thedrive member 8 is pulled in the proximal direction. The axial loadexerted on the lead screw 5 is then compensated by the drive feature ofthe lead screw nut 7 engaging the relevant stop feature, particularlycontacting the essentially flat portion of the relevant recess. Thisacts to lock the rotation of the lead screw 5 rather than rotate it,because the lead screw nut 7 is rotationally locked to the body 1 atleast during the operations of setting and dispensing a dose.Essentially, the flat surfaces on the screw thread 6 are designed toprevent a back-driving of the lead screw 5 during a set operation. Themotion of the lead screw 5 may thereby be restricted to the distaldirection.

FIG. 2 shows an enlarged detailed view of the arrangement of the leadscrew nut 7 and the drive member 8. In the embodiment shown in FIG. 2,the dispense stop feature 19 of the lead screw nut 7 has the shape of atruncated prism. A surface of the dispense feature 19 is oblique to theaxis 4 and faces a corresponding dispense stop feature 20 of the drivemember 8, which is provided with a triangular recess matching thedispense stop feature 19 of the lead screw nut 7. This shape of thedispense stop features 19, 20 is especially favourable, because both arotation and a purely axial movement of the drive member 8 areinhibited. The dispense stop features 19, 20 thus help to ensure thataccurate doses of the drug are dispensed with each device actuation. Toachieve accurate doses, it is essential that the axial and rotationalposition of the drive member 8 relative to the lead screw nut 7 isidentical at the end of each dose delivery. The geometry of the dispensestop features 19, 20 is designed to achieve this.

In the embodiment according to FIG. 2, the dispense stop features 19, 20present the shape of right-angled triangles, with the hypotenuse of eachtriangle arranged oblique to the axis 4 and matching the helix angle ofthe internal thread 18 of the drive member 8, thus forming correspondingcontact surfaces 22, which are oblique to the axis 4. The surfaces thatcorrespond to a side of the triangle are arranged along the axis 4 andare provided as end faces 23 to inhibit a rotation of the drive member 8with respect to the lead screw nut 7 in the direction of rotation of thelead screw 5 during the drug delivery. This means that as the drivemember 8 axially engages with the lead screw nut 7 by means of thedispense stop features 19, 20, it is guided along a helical path untilthe end faces 23 of the triangular dispense stop features 19, 20 engage,thus ensuring that the end position of the drive member 8 is accuratelycontrolled both axially and rotationally. Because the angle of thehypotenuse 22 of the triangular dispense stop features 19, 20 matchesthe helix angle of the thread 18 of the drive member 8, this alsoensures that the lead screw 5 will not be moved by the drive member 8when the dispense stop features 19, 20 are in contact. When the drivemember 8 approaches its specified end position, the dispense stopfeatures 19, 20 force the movement of the drive member 8 into a helixthat matches the helix of the thread 18, so that the movement of thedrive member 8 does not change the position of the lead screw 5. The endof the drug delivery is thus very precisely determined.

FIG. 2 shows an axial opening of the lead screw nut 7. This opening orgap can be used as a guide feature 21, which may provide the rotationallocking of the lead screw nut 7 with respect to the body 1, for example.Instead, other locking means may be provided.

FIG. 3 shows an enlarged detailed view of the distal end of the leadscrew 5. In this embodiment the lead screw 5 comprises a screw thread 6and a further screw thread 16, which are intertwined and are providedwith separate entries (“two-start” thread). The lead screw nut 7 engagesthe screw threads 6, 16 of the lead screw 5. Stop features 17 may beprovided on one screw thread 6 or on both screw threads 6, 16. The pitchof the screw threads 6, 16 can be adapted to the pitch of the thread 18of the drive member 8 in order to provide a desired ratio of the speedsof advancement of the lead screw 5 and the drive member 8.

The dispense stop features 19, 20 of the lead screw nut 7 and the drivemember 8 improve the dose accuracy and prevent an incorrect dosage,which might occur because of mechanical play of the device components.As the dispense stop features 19, 20 can be formed as integral parts ofthe lead screw nut 7 and the drive member 8, the manufacturing isfacilitated and no additional components are necessary.

1-10. (canceled) 11: A drive mechanism for a drug delivery device,comprising: a lead screw, a lead screw nut and a drive member, alignedwith an axis defining an axial direction and an opposite axialdirection, a coupling between the lead screw and the lead screw nutallowing a helical movement of the lead screw with respect to the leadscrew nut at least in the axial direction, the lead screw being coupledwith the drive member, the coupling generating a helical movement of thelead screw with respect to the drive member when the drive member ismoved in the axial direction with respect to the lead screw, and thecoupling being overridden to prevent a helical movement of the leadscrew with respect to the drive member when the drive member is moved inthe opposite axial direction with respect to the lead screw, a dispensestop feature of the lead screw nut, and a dispense stop feature of thedrive member, the dispense stop features interacting and therebypreventing the generation of the helical movement of the lead screw whenthe drive member approaches a specified end position. 12: The drivemechanism according to claim 11, wherein the dispense stop features havecorresponding contact surfaces, which are oblique to the axis. 13: Thedrive mechanism according to claim 11, wherein the dispense stopfeatures have corresponding end faces, which inhibit a rotation of thedrive member with respect to the lead screw nut at least in onedirection. 14: The drive mechanism according to claim 11, wherein thedispense stop feature of the lead screw nut has the shape of a prism ortruncated prism. 15: The drive mechanism according to claim 11, whereinthe dispense stop features guide the drive member with respect to thelead screw nut in a helical movement having the same pitch as thehelical movement of the lead screw with respect to the drive member. 16:The drive mechanism according to claim 11, further comprising: aflexible guide feature of the lead screw, and a screw thread of thedrive member, the coupling of the lead screw with the drive member beingprovided by the flexible guide feature engaging the screw thread, andthe screw thread having the same pitch as the helical movement of thelead screw with respect to the drive member. 17: The drive mechanismaccording to claim 11, further comprising: stop features of the leadscrew, the stop features inhibiting the helical movement of the leadscrew when the drive member is moved in the opposite axial directionwith respect to the lead screw. 18: The drive mechanism according toclaim 11, wherein the drive member and the lead screw nut arerotationally locked. 19: The drive mechanism according to claim 11,wherein the dispense stop feature of the lead screw nut is formed as anintegral part of the lead screw nut, and the dispense stop feature ofthe drive member is formed as an integral part of the drive member. 20:A drug delivery device, comprising: a drive mechanism according to claim1, and a body having a distal end and a proximal end, which are spacedapart in the direction of the axis.